Ultrapure water, that is water with a low concentration of ionic species, is required in many industrial processes. For example, ultrapure water is required in the processing of semiconductor devices to produce defect free devices. Other industries that requires ultrapure water are the pharmaceutical, agricultural, chemistry and food industries. Ultrapure water is also required in nuclear reactors, particularly for the prevention of corrosion. The monitoring of ionic species in water in light water reactors (LWR) such as pressurized water reactors (PWR) and boiling water reactors (BWR), is essential to control corrosion. Monitoring of ionic species in nuclear power plants must be able to monitor very low concentrations, in some applications at concentrations as low as parts-per-trillion (ppt).
Conductometric and photometric detection has been employed to detect the presence of ionic species in water. Currently, virtually all nuclear plants use in-line Ion Chromatography (IC) for routine monitoring of ionic species. The high cost of consumables together with the large sampling volumes that are required and the long analysis times inherent to IC monitoring provides considerable incentive to develop alternative measuring or monitoring methodologies.
One such alternative methodology is Capillary Electrophoresis (CE), which employs 10 to 100 times lower eluent volumes and provides an order of magnitude quicker measurement time (typically less than a three minute compared to 15 to 30 minutes or more for IC). Further, CE requires three to six orders of magnitude less analysts sample, which is particularly attractive to analysis of nuclear reactor water given the escalating radiation waste costs. CE is also capable of simultaneous transition metal cation separations not easily done by IC. A general discussion of CE can be found in the textbook Capillary Electrophoresis of Small Molecules and Ions, Peter Jandik, Gunther Bonn, 1993, Chapter 3 in particular.
Much effort has been focused on the development of CE systems. Jones et al describes in U.S. Pat. No. 5,566,601 a technique for separating, identifying and measuring ions in solution by capillary zone electrophoresis, which provides improved sensitivity and resolution of ionic species. The method involves introducing a sample containing the ionic species into a narrow bore capillary filled with a carrier electrolyte containing a selected visible or UV-absorbing anion or probe. An electrical potential is applied across the capillary column causing the ions to elute according to their mobility. Both ultraviolet (UV) absorbing and UV-transparent ions can be detected and quantitated by UV/visible photometric monitoring. They suggest using as the light-absorbing anion, one selected from molybdate, tungstate, ferrocyanide, ferricyanide, bromide, iodide and dichromate as examples.
As has been discussed in the literature, one aspect of development of CE systems has focused on the selection of the probe used in CE. When detection of inorganic or small molecular weight organic species (i.e. the “species of interest” or analysts) that lack appreciable absorbance, CE detection is typically performed by indirect detection using a background electrolyte (BGE) containing a UV absorbing species—also referred to as the probe. The UV transparent analysts is detected by displacement of the probe by the analysts.
It is suggested that a number of factors are important in selection of the probe. First the electrophoretic mobility (μ) of the probe is considered and should be closely similar to the mobilities of the analytes of interest. Second the minimum detectable concentration clim of the CE system is considered, and is established by the following relationship:clim=cm/(TR×DR),  (1)where cm is the concentration of the probe, TR is the transfer ratio, and DR is the dynamic reserve which is a measure of the ratio of the signal to noise for a given signal. The transfer ratio is the number of equivalents of the probe ions that will be displaced by each equivalent of analysts ions. For example, a common probe pyromellitic acid, when totally ionized, has a transfer ratio of 0.25 for mono-valent ions, and 0.5 for divalent ions. The dynamic reserve is given by the relationship:DR=(ε·L·cm)/AN  (2)where ε is the molar absorptivity of the probe, L is the path length of the light through the capillary, and AN is the absorbance noise level which is a function of the detector wavelength. Substituting for DR in Equation 2 into Equation 1, the following expression for the minimum detectable concentration is obtained:clim=AN/(ε·L·TR)  (3)A general discussion of these principles and equations may be found in Capillary Electrophoresis of Small Molecules and Ions, Peter Jandik, Gunther Bonn, 1993, Chapter 3, and particularly at pages 134-150.
Many probes have been taught in the prior art. For example, U.S. Pat. No. 5,128,005 teaches using a chromate ion. U.S. Pat. No. 5,156,724 teaches using a UV-absorbing amine or heterocyclic sulfate compound. U.S. Pat. No. 5,104,506 teaches using a chromate salt and an alkyl quaternary ammonium salt. Another probe which has been used in the prior art for indirect detection in CE is the aromatic compound pyromellitic acid.
In general, a number of criteria for a successful probe have been discussed in the prior art. Specifically, it is important for the mobility of the analysts to match the mobility of the probe. High molar absorbtivity of the probe is also important. The detector wavelength at which high molar absorbtivity is exhibited by the probe is another important criteria. For example, it is beneficial for the detector wavelength to be a value that minimizes the AN, and which avoids conflict with absorbance bands of the analytes. Finally, the solubility of the anion probe is important when reverse electro-osmotic flow (EOF) modifiers are used in order to avoid precipitation. For probes with low molar absorbtivity, higher probe concentrations are required to obtain satisfactory minimum detectable analysts concentration. The higher probe concentrations can result in potential problems by “probe induced” precipitation of the dynamic EOF modifiers within the capillary. A general discussion of these and other criteria can be found for example in James S. Fritz, Recent developments in the separation of inorganic and small organic ions by capillary electrophoresis; J. of Chromatography A, 884 (2000) 261-275; and Philip Doble, Miroslav Macka, Paul R. Haddad, Design of background electrolytes for indirect detection of anions by capillary electrophoresis, Trends in Analytical Chemistry, vol. 19, no. 1, 2000, pgs 10-17.
While much effort has been focused on the development of probes, the prior art probes available to date are not entirely satisfactory. Contrary to the desired criteria as discussed above, it turns out that many of the prior art probes exhibit strong absorbance in the low UV spectrum, i.e. the detection wavelength, and this is the very region where the analytes or species of interest also exhibit strong absorbance. This similarity in absorbance of the analytes of interest makes indirect detection of analytes difficult. Also, the absorbance noise is elevated at these lower wavelengths, thereby decreasing dynamic reserve. In addition, the minimum detectable concentration clim is often too high for many applications in the semiconductor, nuclear power and other industries due mostly to low molar absorbtivity ε at otherwise optimum detection wavelengths. Further, multi-ionized probes such as pyromellitic acid have low transfer ratios which lead to higher minimum detectable concentrations. Additionally, such probes are typically useful in detecting only a small range of analytes, and are not applicable to a wide range of applications. Accordingly, continued development of improved probes and CE methods are of interest.